Στον ενικό λέμε «ο φοιτητής», όχι «οι φοιτητές».

It means in the singular.

More literally:

• στον = σε + τον = in/to the
• ενικό = singular

So στον ενικό is short for στον ενικό αριθμό, literally in the singular number.

Because the full hidden phrase is στον ενικό αριθμό.

The noun αριθμό is:

• masculine
• singular
• accusative

So the adjective must agree with it, which gives ενικό.

Compare:

• ο ενικός αριθμός = the singular number
• στον ενικό αριθμό = in the singular number

Greek adjectives change form to match the noun they describe.

Because Greek often leaves out words that are obvious from context.

In grammar explanations, στον ενικό is a very common shortened way of saying:

• στον ενικό αριθμό = in the singular
• στον πληθυντικό (αριθμό) = in the plural

So this is just natural ellipsis, not a special rule.

Greek often uses we in explanations where English might use:

• we say
• you say
• one says
• or even just the passive

So λέμε here is not necessarily talking about a specific group of people. It is a very natural teaching/explaining way to say the correct form is...

Because they are being mentioned as word forms, not used as normal parts of the main sentence.

The outer sentence is:

• Στον ενικό λέμε ... όχι ...

Inside it, ο φοιτητής and οι φοιτητές are quoted examples of the form you use.

Also, Greek very often gives a noun together with its article in the nominative, because that shows important information such as:

• gender
• number
• the basic dictionary form

So ο φοιτητής is the standard citation form for student.

They are both definite articles, but they mark different numbers.

• ο = the for masculine singular nominative
• οι = the for plural nominative

So:

• ο φοιτητής = the student
• οι φοιτητές = the students

A learner should also know that οι is used for other nominative plural nouns too, not only masculine ones.

Because φοιτητής is a masculine noun of a common pattern:

• singular: -τής
• plural: -τές

So:

• ο φοιτητής = the student
• οι φοιτητές = the students

This is a very common noun type in Greek. Other nouns can follow different patterns, but this one is regular.

No. In Modern Greek, οι is pronounced ee /i/.

So:

• ο sounds like o
• οι sounds like ee

That means:

• ο φοιτητής is roughly o fi-ti-TIS
• οι φοιτητές is roughly i fi-ti-TES

This pronunciation often surprises English speakers, because the spelling οι looks like it should sound like English oi, but it does not.

Yes, absolutely. It is a perfectly correct Greek phrase. It just is not correct in the singular.

This sentence is saying:

• when you want the singular form, say ο φοιτητής
• not οι φοιτητές, because that is plural

So οι φοιτητές is correct when you mean the students.

όχι means not, and here it is used to correct or reject the wrong option.

The pattern is:

• λέμε X, όχι Y = we say X, not Y

This is a very common way to correct someone in Greek.

For example, a teacher might say:

• Λέμε ... , όχι ...
• γράφουμε ... , όχι ...

So in this sentence, όχι simply introduces the incorrect form.

You could say:

Στον πληθυντικό λέμε οἱ φοιτητές, όχι ο φοιτητής.

That means In the plural we say the students, not the student.

So the pair is:

• στον ενικό = in the singular
• στον πληθυντικό = in the plural

These are two very basic grammar terms worth memorizing early.